Certain prosthetic heart valves incorporate an expandable stent body and valve elements such as prosthetic valve leaflets mounted to the stent body. Valves of this type may be implanted in the heart by advancing the valve into the body of the patient with the stent body in a collapsed condition in which the stent body has a relatively small diameter. Once the valve is positioned at the desired implantation site, the stent body is brought to an expanded condition in which the stent body bears on the surrounding native tissue and holds the valve in place. The valve acts as a functional replacement for the diseased native valve. Thus, the valve elements inside the stent body permit blood flow in the antegrade direction but substantially block flow in the opposite, retrograde direction. For example, a prosthetic valve may be advanced to a site within a diseased native aortic valve percutaneously through the arterial system and into the aorta to the native aortic valve. In a transapical placement, a prosthetic valve may be advanced through an incision in the apex of the heart and through the left ventricle to the native aortic valve. Other approaches through other access sites can be used. Once the prosthetic valve is in place, it permits flow from the left ventricle into the aorta when the left ventricle contracts during systole, but substantially blocks retrograde flow from the aorta into the left ventricle during diastole.
There are significant challenges in the design of an expandable stent body and valve. For example, the stent body desirably can be collapsed to a relatively small diameter to facilitate advancement into the body. However, the stent body must be capable of expanding to an operative, expanded condition in which the stent body securely engages the surrounding native tissues to hold the valve in place. The valve should form a good seal with the surrounding native tissues to prevent leakage around the outside of the prosthetic valve, commonly referred to as perivalvular leakage. The stent body, in its expanded, operative condition, desirably does not apply excessive forces to the annulus of the native valve. Excessive forces on the annulus of the native aortic valve can disrupt the electrical conduction system of the heart and also can impair the functioning of the mitral valve. These issues are complicated by the fact that the native valve leaflets ordinarily are left in place when an expandable prosthetic valve is implanted. The diseased native valve leaflets and other diseased tissues may present an implantation site which is irregular. For example, patients with calcified or stenotic aortic valves may not be treated well with the current collapsible valve designs, and may encounter problems such as (1) perivalvular leakage (PV leak), (2) valve migration, (3) mitral valve impingement, (4) conduction system disruption, etc., all of which can lead to adverse clinical outcomes. To reduce these adverse events, the optimal valve would seal and anchor adequately without the need for excessive radial force that could harm nearby anatomy and physiology.
As the patient population becomes younger with less tissue calcification, the use range of prosthetic heart valves expands to larger sizes and thus greater forces. Additionally, younger patients may be more likely to require bicuspid valve replacements and may have larger, more diseased, and more elliptically shaped aortic valves. Stent designs without an aortic section, as well as stents that need to anchor in the aorta, will need to anchor and support post deflection of the valve as it pulls inward from pressure closing the valve. If a valve is not designed to function in these environments, it may result in several types of adverse events described above since distortion of the valve can result in the loss of valve coaptation, loss of valve apposition, and overall higher stresses. In these environments, anchoring will become even more important to ensure valve coaptation in most all configurations with a reduction of high stresses in the critical areas of the prosthesis while maintaining valve apposition with native anatomy.
Numerous prosthetic valve and stent body designs have been proposed. However, despite all of the attention devoted to such designs, still further improvements would be desirable.